There has been proposed a manufacturing apparatus comprising a rubber extruder for continuously discharging rubber, and a pair of upper and lower calender rolls for rolling the discharged rubber in order to manufacture an unvulcanized rubber strip. (see, the following Patent Document 1 for example) Each of the calender rolls has a roll main body which is driven to rotate, and a cylindrical outer layer fixed to the outside thereof. The roll main body is made of a metal, whereas the outer layer is made of a ceramic. Such outer layer has an advantage such that unvulcanized rubber is hardly adhered thereto as compared with those made of metals.    Patent Document 1: Japanese unexamined Patent Publication No. 2011-194737